Capacitive-discharge system for internal combustion engines

ABSTRACT

The capacitive-discharge system for internal combustion engines comprises a stepup transformer having primary and secondary windings. The capacitor is placed in series with the primary winding. A DC power source including a push-pull full wave oscillator circuit is provided to charge the capacitor. The capacitor is discharged through the primary winding in timed sequence with an operating engine to induce a voltage in the secondary winding sufficient to fire a spark plug.

United States Patent Inventor Alfred Plume, Jr.

Taylor, Mich.

Appl, No. 843,489

Filed July 22, 1969 Patented June 1, 1971 Assignee Mallory ElectricCorporation Detroit, Mich.

CAPAClTIVE-DISCHARGE SYSTEM FOR INTERNAL COMBUSTION ENGINES PrimaryExaminer-Laurence M. Goodridge Attorney-Whittemore, Hu1b'ert& BelknapABSTRACT: The capacitive-discharge system for internal combustionengines comprises a stepup transformer having primary and secondarywindings. The capacitor is placed in 3 Claims 3 Drawing series with theprimary winding. A DC power source including US. Cl [23/148, a push pullfull wave oscillator circuit is provided to charge 315/209 thecapacitor. The capacitor is discharged through the prima- Int. Cl F02p3/06 ry winding in timed sequence with an operating engine to in- Fieldof Search 123/148 E; duce a voltage in the secondary winding sufficientto fire a 315/209, 214 spark plug.

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ALFRED PLUME, JR. QM a 7; WWW M ATTORNEYS CAPACITIVE-DISCHARGE SYSTEMFOR INTERNAL COMBUSTION ENGINES BACKGROUND OF THE INVENTION Inconventional inductive-discharge ignition systems, the coil develops thehigh voltage necessary to cause the vehicle spark plugs to fire.Capacitive-discharge ignition systems have been previously proposed.Capacitive-discharge systems have several advantages over conventionalsystems. One advantage is that such a system has the ability to causefiring of defective spark plugs. Additionally, in such systems thecapacitor is not discharged as often when operating at low speeds as itis when operating at high speeds. This allows a relatively long time inwhich the capacitor achieves a full charge. This is advantageous in thatit results in improved cold-weather starting and reduces powerconsumption by the ignition system while starting.

The present invention provides an improved version of acapacitive-discharge ignition system. In the present system, acontrolled rectifier is provided as the switching device to causedischarge of the capacitor. The controlled rectifier is pulsed toconduct by means of a one-shot circuit. The signal from the one-shotcircuit is carefully controlled so that it will be sharply applied atthe precise desired instant and the circuit is protected from straysignals being inadvertently applied to cause misfiring. Additionally,protective means are provided to shield the controlled rectifier fromhigh peak inverse voltages.

SUMMARY OF THE INVENTION The capacitive-discharge system is provided foran internal combustion engine/It comprises a stepup transformer havingprimary and secondary windings. A capacitor is provided in series withthe primary winding. A DC power source including a push-pull full waveoscillator circuit is provided to charge the capacitor. Discharge meansincluding a controlled rectifier are provided to discharge the capacitorthrough the primary winding in timed sequence with an operating engineto induce a voltage in the secondary winding sufficient to fire a sparkplug. Pulse producing means are provided. A first transistor is coupledbetween the pulse-producing means and the gate of the controlledrectifier for biasing the controlled rectifier to conduct in the desiredtimed sequence with an operating engine. A diode is provided between theoutput of the first transistor and the gate of the controlled rectifier.The diode conducts only when a minimum predetermined voltage is appliedthereto. The first transistor is valued to provide the necessary voltagewhen it is fired to conduct. A second transistor is connected betweenthe output of the first transistor and ground. Means are provided tobias the second transistor to a conducting state when the firsttransistor is not biased to conduct by a signal from the pulse-producingmeans and to a nonconducting state when the first transistor is biasedto conduct by a signal from the pulse-producing means.

IN THE DRAWINGS:

FIG. l is a schematic view of one embodiment of an electrical switchingcircuit for a vehicle ignition system in accordance with the presentinvention.

FIG. 2 is a schematic view of the push-pull full wave oscillator circuitutilized as a power source; and

FIG. 3 is a graph illustrating operation of the oscillator circuit.

Referring to FIG. 1, it will be noted that a stepup transformer I isincluded in the circuit for providing a voltage sufficiently high tofire a spark plug of a vehicle engine. The transformer comprises aprimary winding 12 and a secondary winding 14. The high voltage outputof the winding 14 is utilized by the distributor (not shown) of theignition system for firing the vehicle spark plugs.

A lead 16 extends from one side of the primary winding 12 and isconnected to one side of a capacitor 18. A lead 20 extends from theother side of the capacitor I8 and is connected at Z to a DC powersource (FIG. 2) which is capable of supplying a relatively high voltage,in the neighborhood of 225- 400 volts.

In operation of the system, the capacitor 18 is first charged to thevoltage of the power supply. The capacitor 18 is then suddenlydischarged through the primary winding 12 whereby the voltage applied tothe primary winding rises to the full voltage of the capacitor, in thepresent instance about 400 volts, in a very short period of time, forexample, two microseconds. The voltage induced in the secondary winding14 is sufficient to cause the spark plugs to fire.

The present invention is concerned with the circuitry for causing thecapacitor 18 to charge and discharge and the DC power source. Thiscircuitry includes a solid-state controlled rectifier 22 which isprovided in a lead 24 which extends between the lead 20 and a groundlead 26. The rectifier 22, which may be a silicone-controlled rectifier,has an anode 28, a cathode 30 and a gate 32. As is well known, acontrolled rectifier is a solid-state four-layer device. In its normalstate, the controlled rectifier actsas an open circuit that will notpass current. When an appropriate voltage or current pulse is applied tothe gate electrode, it will cause the controlled rectifier to be forwardbiased .to permit current flow. Application of the proper polarityvoltage to the controlled rectifier will allow electrons to flow fromthe cathode to the anode. Reversal of the voltage polarity results inthe controlled rectifier being an open circuit. Similarly, when thecontrolled rectifier is conducting, application of a reverse polarity tothe gate electrode will place the controlled rectifier in its originalstate of an open circuit. Thus, the controlled rectifier can act as acontrolled switching diode capable of being switched on and off byapplication of voltages of appropriate polarity.

In the present invention, the gate 32 is connected to circuitry whichprovides a pulse which is in timed relation to the engine speed. Thepulse,-in the present case, derives from conventional breaker points 34which are opened and closed in timed relation to the speed of thedistributor shaft. However, the signal may be derived from any suitablesource from the engine. One satisfactory method for providing a pulse isdisclosed in copending application Ser. No. 447,004, filed Apr. 9, 1965,now U.S. Pat. No. 3,408,993.

The pulsing circuit includes a conventional vehicle battery 36 thenegative terminal of which is grounded at 38. A lead 40 extends from thepositive terminal of the battery. An ignition switch 42 is provided inlead 40 for turning the ignition system on and off.

A lead 44 extends between the leads 40 and 26. A current limitingresistor 46 and the breaker points 34 are provided in the lead 44. Itwill be noted that the lead 26 is grounded at 48. Thus, when the breakerpoints '34 are closed, current will flow therethrough from the battery36 to ground. A lead 50 extends from a point between the resistor 46breaker points 34 to the base 52 of a transistor 54. The transistor 54forms a portion of what is termed a one-shot circuit" which receivespulses generated by the opening and closing of the breaker points 34which are proportional to the speed of the engine. The oneshot circuitconverts these pulses into a square'wave signal which operates theswitching means for charging and discharging the capacitor 18.

A pair of resistors 56, 58 are provided in the lead 50. A lead 60extends from a point between the resistors 56, 58 to the ground lead 26.A capacitor 62 is provided in lead 60. Another capacitor 63 is providedin a lead 65 which extends from the other side of resistor 58 to groundlead 26. The resistor 56 and capacitors 62, 63 form a filter whichremoves any oscillations of the input voltage. The resistor 58 forms abiasing resistor for the transistor 54. The transistor 54 is connectedin the common emitter configuration, the emitter 66 being connected toground lead 26 by lead 68 and the collector 70 of the transistor beingconnected to the potential supply line 40 by a lead 72. A load resistor74 is provided in the lead 72.

The one-shot circuit also includes a second transistor 64. The emitter76 is connected to ground lead 26 by a lead 78. The collector 80 isconnected to the potential supply line 40 by a lead 82. A load resistor84 is provided in the lead 82. The base 86 is coupled by a capacitor 88in lead 90 to the collector 70 of the transistor 54. A lead 92 extendsfrom the lead 90 into connection with the potential supply line 40. Aresistor 94 is provided in lead 92. The resistor 94 functions as abiasing resistor for the transistor 64 and also as a time constantdevice for controlling operation of the transistor.

The collector 80 of the transistor 64 is connected by a lead 96 to thebase 52 of transistor 54. A resistor 98 is provided in lead 96 betweenthe lead 82 and the base 52. The resistor 98 acts as a feedback devicefor feeding back part of the output signal from transistor 64 to thebase of the transistor 54.

The output of the transistors 54, 64, forming the one-shot circuit, isfed to a second pair of transistors 100, 102. These transistors formpart of a switching circuit which includes the controlled rectifier 22.

The base 104 of the transistor 100 is connected to the collector oftransistor 64 by means of the lead 96 which is connected to the lead 82.The collector 106 of transistor 100 is connected to the potential supplyline 40 by a lead 108. The emitter 110 of transistor 100 is connected tothe collector 112 of transistor 102 by a lead 114. A resistor 116 isprovided in lead 114. The resistor 116 is a current limiting device andfunctions to prevent reverse or leakage current from damaging thecomponents.

The base 118 of the transistor 102 is connected to the base 86 oftransistor 64 by means of a lead 120. The emitter 122 of transistor 102is connected to ground lead 26 by a lead 124.

A lead 126 extends from the lead 114 from a point between resistor 116and emitter 112 into connection with the gate 32 of the controlledrectifier 22. A diode 128 is provided in the lead 126. The diode 128will not conduct until the voltage thereacross reaches a predeterminedlevel. For example, in one circuit, the diode was designed to conduct at0.67 volts. The proper voltage for conduction of the diode 128 isprovided at the exact point of turning one of the transistors 100, 102off, and simultaneously turning the other of these transistors on, aswill be later described.

The diode 128 emits a square-wave pulse to the controlled rectifier 22to cause conduction of this rectifier. A filter capacitor 129 isprovided in level 131 between gate 32 and ground lead 26. The capacitor129 functions to filter out fake frequencies resulting from point bounceand any other transient frequencies which may appear at the gate 32.

A diode 130 is provided in the lead 24 between the anode 28 of therectifier 22 and the lead 20. The diode 130 is a high voltage, highcurrent, high speed device. The voltage drop thereacross is very rapid.This prevents high peak inverse voltage being applied across therectifier 22 and thus is a protective device for the rectifier.

A pair of diodes 132, 133 are provided in a lead 134 which extendsbetween leads 20 and 26 across the rectifier 22. It will be noted thatthe diodes 132, 133 are oriented to conduct in the opposite directionwith respect to the diode 130 and rectifier 22. The function of thediodes 132, 133 is to shut out any high voltage negative pulses whichare reflected from the transformer or any other source. As is wellknown, such transformers tend to ring after the initial surge of currenthas flowed therethrough upon discharge of the capacitor 18. Such ringingmay cause high voltage negative pulses which are of sufficient magnitudeto cause damage. A resonant circuit is placed in series with the diodes132, 133. The circuit consists of an inductance 136 provided in the lead134 and stray capacitance of the circuit which is present across theinductance 136. The resonant circuit functions to provide a signal tocause the controlled rectifier 22 to discontinue conduction after thecapacitor 18 has been discharged. The circuit also damps the reflectedpulses from the transformer 10 after the transformer has been activatedto produce the required voltage for firing a spark plug, and rechargesthe capacitor 18 to some degree.

Operation of the entire circuit of FIG. 1 may now be understood. Thecircuit will be considered in its quiescent condition just before aninput pulse is applied to the transistor 54. in this condition,transistor 64 is biased for conduction, and a voltage just above groundpotential appears at the collector of transistor 64. This voltage isapplied to the base 52 of transistor 54 via the resistor 98.Consequently, the base 52 of transistor 54 is not sufficiently positiveto turn transistor 54 on. Transistor is also in its off condition. As aresult of the conduction of transistor 64, the capacitor 88 is chargedto the polarity indicated.

The transistor 102 is also biased to conduct at this time. It will benoted that the collector 112 is connected to the potential supply line40 through transistor 100. Therefore, the only current which will flowthrough the collector-emitter circuit of transistor 102 is that whichleaks through transistor 100 while it is in the off state. The result ofthis is that the battery voltage always appears across transistor 100and when this transistor is biased to conduct, the peak voltage of thesignal from the one-shot circuit is reached very quickly and applied tothe gate of controlled rectifier virtually instantaneously.

When a positive pulse is applied to the base oftransistor 54, as byopening the points 34, the transistor 54 is turned on. Its collector 70then foes to a potential just above ground potential. The voltage acrossthe capacitor 88 suddenly goes negative, and the capacitor begins todischarge through the conducting transistor 54. The voltage on capacitor88 turns off the transistor 64 and causes its collector 80 to gopositive. This turns the transistor 100 to the on state. The positivevoltage accruing at the collector 80 of transistor 64 is also appliedvia the resistor 98 to the base 52 of transistor 54 and holds thattransistor on even after the input pulse has terminated.

When the voltage across the capacitor 88 suddenly goes negative, and thecapacitor begins to discharge through the conducting transistor 54, thevoltage at capacitor 88 also turns off the transistor 102.

It will thus be appreciated that initially transistors 64 and 102 areconducting while transistors 54 and 100 are not conducting. Upon theapplication ofa positive pulse to the base of transistor 54, thetransistor 54 begins to conduct. Simultaneously, transistors 64 and 102are turned off while transistor 100 is turned on. Conduction oftransistor 100 causes the required voltage to appear across diode 128thus causing this diode to conduct. lt will be appreciated that thisdiode will conduct at the exact point of turning the transistor 100 onand turning he transistor 102 off. Conduction of diode 128 causes thecontrolled rectifier 22 to begin conducting thus discharging thecapacitor 18 resulting in a high induced voltage in secondary winding 14to cause sparking ofa spark plug.

After capacitor 88 has fully discharged, the base 86 of transistor 64 isagain made positive and transistor 64 begins to conduct again. Thecollector 80 of transistor 54 then goes negative nearly to groundpotential, and this potential is again applied by resistor 98 to thebase 52 of transistor 54 turning that transistor off. Transistor 100 isalso turned off and remains off so long as the near-ground potentialremains at the collector 80 of transistor 64. At the same time,transistor 102 is again caused to conduct as the capacitor 18 isrecharged bringing the entire system back to the original state.

The controlled rectifier 22 will remain in the on condition so long asthe transistor 100 conducts. The length of time that the transistor 100conducts is determined by the R-C time constant of resistor 94 andcapacitor 88. The capacitor 88 will, of course, begin to charge to thebattery voltage through the resistor 94 as soon as the transistor 54begins to conduct. The time necessary for this to occur is such that avoltage of sufficient value will have been developed in the secondarywinding 14.

Referring now to FIG. 2, the power circuit 142 includes an amplifierwhich comprises a pair of NPN transistors 144, 146, connected incircuitry to form an amplifier of the push-pull full-wave type. Thetransistors 144, 146 are connected in the common emitter configuration.The emitters 148, 150 are connected via leads 152, 154 to a lead 156.The lead 156 is grounded at 158.

The lead 156 is connected to the center of a lead 160 which extends inone direction to one side of a bias winding 162 and in the otherdirection to one side of a second bias winding 164. The other side ofwinding 162 is connected to the base 166 of transistor 144 via lead 168.A load resistor 170 is provided in lead 168. The other side of thewinding 164 is connected to the base 172 of transistor 146 via lead 174.A load resistor 176 is provided in lead 174.

The negative side of the battery 36 is connected at point Y. Thepositive side of the battery 36 is connected to the center of a lead 178at the point X. The lead 178 is connected between leads 168, 174. Aresistor 180, 182 is provided in the lead 178 on either side of thebattery connection X. A lead 184 extends from lead 178 to the center ofthe primary winding 186 of stepup transformer 188. One side of thewinding 186 is connected to the collector 190 of transistor 144 via lead192. The other side of the primary winding 186 is connected to thecollector 194 of transistor 146 via lead 196.

The secondary winding 198 of the transformer 188-has a center tap ground200. One side of the winding 198 is connected to the positive terminalof a junction diode 202 via lead 204. The negative terminal of diode 202is connected to the positive terminal of a second diode 206 via lead208. A lead 210 extends from the negative terminal of diode 206 and isconnected at Z to the lead 20. Similarly, a lead 212 extends from theother side of the winding 198 to the positive terminal of a diode 214.The negative terminal of diode 214 is connected to the-positive terminalof a diode 216 via lead 218. A lead 220 extends from the negativeterminal of diode 216 into connection with lead 210. The diodes 202,206, 214, 216 serve as a rectifying circuit. A pair of diodes is placedin series in each of the circuits in order to prevent breakdown of thediodes as a result of peak inverse voltage. v

A lead 222 extends from lead 210 to ground. The lead 222 is connected tolead 210 between the connection of lead 220 and the point Z. A capacitor224 is provided in lead 222.

Another lead 226 extends from lead 210 in parallel with lead 222. Thelead 226 is also grounded. A resistor 228 is provided in lead 226.

An inductance 230 is provided in lead 210 beyond the connection thereofwith lead 222. The inductance 230 serves to stop spikes in lead 210 ineither direction A diode 232 is provided between inductance 230 and theconnection point Z. The diode 232 functions to prevent application ofreverse voltage on the circuit 142.

Operation of the circuit 142 may now be understood. The transistors144,146 in conjunction with the transformer 188 form a DC to ACinverter. When the battery voltage is applied by closure of switch 42,one of the transistors will conduct while the other will go into cutoff.Assuming that transistor 144 conducts, the expanding field in theprimary winding 186 of the transformer 188 caused by the conduction oftransistor 144 will develop a forward bias in the base-to-emitterwinding 162 of transistor 144 and will maintain transistor 144 inconduction.

When saturation is reached, the magnetic field becomes stationary andthere are no longer any moving lines of force to maintain theinduced-bias voltage. The transistor 144 will then cease conducting andthe magnetic field of transformer 188 will collapse. As this fieldcollapses, it induces a voltage of the opposite polarity that placestransistor 146 in conduction and transistor 144 is cut off. Withtransistor 146 conducting through the opposite half of the primarywinding 186, the magnetic field reverses polarity. As the field reachessaturation, the cycle repeats. Transistor 146' becomes cut off andtransistor 144 conducts and the magnetic field of primary winding 186 ofthe transformer 188 reverses polarity. Consequently, transistors 144,146 act as an oscillator with the voltage on the secondary winding 198of the transistor 188 appearing as a sine wave.

The high AC voltage developed in the secondary winding 198 of thetransformer 188 is rectified by diodes 202, 206 and 214, 216. Theresistor 228 and capacitor 224 along with the inductance 230 function tosafeguard the oscillator circuit.

The resistor 228 provides a load for the charge on the capacitor 224which cyclically discharges at certain times in operation of the circuitas, for example, when an internal combustion is being started and thevehicle spark plugs are not yet firing. The time constant prevents thevoltage of the circuit from going above, for example, 600 volts when2,000 volts are available at the output of the transformer 188. Thecircuit comprising resistor 228 and capacitor 224 eventually fade outafter the spark plugs of the vehicle engine begin firing because theimpedance of the remaining circuit falls below the impedance of theresistor. The capacitor 224, in combination with the inductance 230, isa safety device to prevent the oscillator circuit from ever seeing afull short as it might, for example, when the capacitor 18 discharges.This prevents complete stopping of the oscillator which wouldnecessitate restart.

The desirability of this arrangement may be understood by study of thecurve illustrated in FIG. 3. The curve represents engine speed plottedagainst output voltage which is sustained by the distributor. As will benoted, a maximum of 600 volts is possible at zero r.p.m. This would bethe condition at startup of the vehicle. With the engine speed at 200rpm, the voltage would be approximately 400 volts. As engine speedincreases, the voltage drops somewhere to 225 volts and will remainrelatively constant up to speeds of over 10,000 rpm. In one embodiment,a relatively steady output voltage was available at engine speeds of upto l9,000 r.p.m., which is a circumstance only rarely encountered in aninternal combustion engine. Thus, the sparking voltage is sufficientover a broad spectrum of engine speeds.

What I claim as my invention is:

1. In a capacitive-discharge system for internal combustion enginescomprising a stepup transformer having primary and secondary windings, acapacitor in series with the primary winding, a DC power source tocharge the capacitor, discharge means including a controlled rectifierto discharge the capacitor through the primary winding in timed sequencewith an operating engine to induce a voltage in the secondary windingsufficient to fire a spark plug, pulse-producing means, a firsttransistor coupled between said pulse-producing meansand the gate ofsaidcontrolled rectifier for biasing the controlled rectifier to conductin timed sequence with an operating engine, a diode between the outputof the first transistor and said gate, said diode conducting only when aminimum predetermined voltage is applied thereto, said first transistorbeing valued to provide the necessary voltage when it is biased toconduct, and a second transistor connected between the output of thefirst transistor and ground, means biasing the second transistor to aconducting state when the first transistor is not biased to conduct by asignal from the pulseproducing means and to a nonconducting state whenthe first transistor is biased to conduct by a signal from thepulseproducing means. v

2. A capacitive-discharge system as defined in claim 1, and furthercharacterized in that said pulse-producing means includes a one-shotcircuit, said one-shot circuit comprising third and fourth transistorscross-coupled by means including a capacitor for translating inputpulses to a square wave signal, said first transistor being connected tothe collector-emitter circuit of said fourth transistor and biased toconduct when said third transistor conducts, said second transistorbeing connected between said third and fourth transistors and biased toconduct when said fourth transistor conducts.

3. In a capacitor-discharge system for internal combustion enginescomprising a stepup transformer having primary and secondary windings, afirst capacitor in series with the primary winding, a DC voltageamplifier including an oscillator, a second capacitor connected to theoutput of the amplifier to be charged thereby, a resistor in parallelwith said capacitor, said resistor providing a discharge path for thecapacitor'during operation of the amplifier under no-load conditions,the output of said DC power source being connected to said firstcapacitor to charge said first capacitor, discharge means to dischargethe first capacitor to the primary winding in timed' sequence with anoperating engine to induce a voltage in the secondary winding sufficientto fire a spark plug, pulse producing means, a first transistor coupledbetween said pulse producing means and the discharge means for actuatingsaid discharge means, a voltage device between the output of the firsttransistor and said discharge means, said voltage device conducting onlywhen a minimum predetermined voltage is applied thereto, said firsttransistor being valued to provide the necessary voltage when biased toa conducting state, and a second transistor connected between the outputof the first transistor and ground, means biasing the second transistorto a

1. In a capacitive-discharge system for internal combustion enginescomprising a stepup transformer having primary and secondary windings, acapacitor in series with the primary winding, a DC power source tocharge the capacitor, discharge means including a controlled rectifierto discharge the capacitor through the primary winding in timed sequencewith an operating engine to induce a voltage in the secondary windingsufficient to fire a spark plug, pulse-producing means, a firsttransistor coupled between said pulse-producing means and the gate ofsaid controlled rectifier for biasing the controlled rectifier toconduct in timed sequence with an operating engine, a diode between theoutput of the first transistor and said gate, said diode conducting onlywhen a minimum predetermined voltage is applied thereto, said firsttransistor being valued to provide the necessary voltage when it isbiased to conduct, and a second transistor connected between the outputof the first transistor and ground, means biasing the second transistorto a conducting state when the first transistor is not biased to conductby a signal from the pulse-producing means and to a nonconducting statewhen the first transistor is biased to conduct by a signal from thepulse-producing means.
 2. A capacitive-discharge system as defined inclaim 1, and further characterized in that said pulse-producing meansincludes a one-shot circuit, said one-shot circuit comprising third andfourth transistors cross-coupled by means including a capacitor fortranslating input pulses to a square wave signal, said first transistorbeing connected to the collector-emitter circuit of said fourthtransistor and biased to conduct when said third transistor conducts,said second transistor being connected between said third and fourthtransistors and biased to conduct when said fourth transistor conducts.3. In a capacitor-discharge system for internal combustion enginescomprising a stepup transformer having primary and secondary windings, afirst capacitor in series with the primary winding, a DC voltageamplifier including an oscillator, a second capacitor connected to theoutput of the amplifier to be charged thereby, a resistor in parallelwith said capacitor, said resistor providing a discharge path for thecapacitor during operation of the amplifier under no-load conditions,the output of said DC power source being connected to said firstcapacitor to charge said first capacitor, discharge means to dischargethe first capacitor to the primary winding in timed sequence with anoperating engine to induce a voltage in the secondary winding sufficientto fire a spark plug, pulse producing means, a first transistor coupledbetween said pulse producing means and the discharge means for actuatingsaid discharge means, a voltage device between the output of the firsttransistor and said discharge means, said voltage device conducting onlywhen a minimum predetermined voltage is applied thereto, said firsttransistor being valued to provide the necessary voltage when biased toa conducting state, and a second transistor connected between the outputof the first transistor and ground, means biasing the second transistorto a conducting state when the first transistor is not biased to conductby a signal from the pulse-producing means, and to a nonconducting statewhen the first transistor is biased to conduct by a signal from thepulse-producing means, and an inductor in series with said secondcapacitor, said inductor being located between said first and secondcapacitors and functioning, along with said second capacitor, to preventa full short across the voltage amplifier during discharge of said firstcapacitOr.